The present invention relates to improvements in scanning wheels used in ion implantation process chambers.
Ion implantation is used for doping semiconductor materials to achieve desired conductivity states. A beam of ions of a desired species for implantation is formed and directed at the surface of a semiconductor wafer for implantation therein. In batch processing ion implanters, it is a usual practice to arrange for a batch of semiconductor wafers to be processed simultaneously, and for these wafers to be repeatedly scanned through the ion beam to ensure a homogenous distribution of ions over the surfaces of the wafers.
U.S. Pat. No. 5,389,793 discloses a scan wheel used for scanning a batch of wafers in the process chamber of an ion implanter. The scan wheel disclosed in this U.S. patent specification comprises a central hub and a plurality of separate spoke arms mounted to the central hub, each having at its outer extremity a wafer support element for carrying a semiconductor wafer. The spoked scan wheel is rotated rapidly about an axis to achieve scanning of the wafers through the beam in one co-ordinate direction. At the same time, the axis of rotation of the scan wheel is reciprocated to and fro to achieve scanning of the wafers in a second co-ordinate direction. The combined scanning motions produce a raster scanning of the ion beam across each of the wafers held on the wafer support elements of the scan wheel.
The present invention is concerned with improvements to the design of the scan wheel as disclosed in the above U.S. patent specification. There is a need for increased processing speed, a reduction in the level of contamination in processing chambers, and the ability to handle larger wafers.
In one aspect the invention provides a scan wheel for an ion implanter adapted for carrying a plurality of semiconductor wafers, the scan wheel being rotatable about an axis for scanning wafers carried by the wheel across an ion beam generated in the implanter, said scan wheel comprising a central hub, a plurality of separate spoke arms mounted to said central hub and extending radially outward therefrom, and a plurality of wafer support elements formed on the outer ends of respective said spoke arms and each including a front face for mounting a semiconductor wafer thereon, said spoke arms each having a dimension in the direction of rotation which is substantially less than the corresponding dimension of the wafer support element, and said spoke arms each having a front face extending generally in said direction of rotation and side faces extending rearwardly from said front face, wherein, at least over a predetermined radial distance inwards from an innermost edge of the respective wafer support element, each of said side faces has a major part which does not extend outwards beyond a rearward projection of said front face which, in cross-section, tapers symmetrically inwards, preferably by 7xc2x0, on each side relative to said axis of rotation.
The design of scan wheel using relatively narrow spoke arms supporting the larger wafer support elements reduces the surface area of the scan wheel which is irradiated by the ion beam during that part of the scanning process, called maximum over scan, when the axis of rotation of the scan wheel is at its closest to the ion beam so that the ion beam is just clear of the innermost edges of the wafers mounted on the wafer support elements. This is desirable not only to reduce the thermal loading on the scan wheel resulting from the energy of the ion beam dissipated on the spoke arms, in addition to the energy dissipated on the semiconductor wafers supported by the support elements, but also to reduce the area of the scan wheel from which contaminants may be sputtered during irradiation by the ion beam. Any such sputtered contaminants may produce contamination on the surface of the semiconductor wafer being processed and increase unwanted material on the processed wafers.
It is established practice to arrange for the axis of rotation of the scan wheel to be adjustable relative to the direction of the ion beam, with the effect that the angle of implant of ions into the semiconductor wafers can be correspondingly adjusted. In implanters available currently, implant angle is typically adjustable between plus and minus 10xc2x0 to the normal. An arrangement of this kind is shown in U.S. application Ser. No. 08/626,746.
In the design of process chamber illustrated, adjusting the axis of rotation of the scan wheel to provide a desired implant angle has the effect of adjusting the angle of the semiconductor wafers as they traverse the ion beam about an axis generally parallel to the spoke arm on which the semiconductor wafer is mounted. As a result, when the implant angle is set to be other than normal to the wafer surface, the spoke arms traverse the ion beam also with a corresponding longitudinal angle of rotation.
By arranging for the side faces of the spoke arms not substantially to extend beyond a tapered rearward projection of the front face of the spoke arm, the side faces can be shadowed by the front face of the respective arm for a range of implant angles on either side of normal. This is important since the total area of the spoke arms irradiated by the ion beam during this part of the scan is thereby reduced. Indeed, for a taper of 7xc2x0 and implant angles of less than about 7xc2x0, the side faces of the spoke arms should scarcely be irradiated by the ion beam.
It is also known from U.S. Pat. No. 4,937,206 to coat areas of a scan wheel which are liable to be irradiated by the ion beam with a process compatible material, typically silicon. This minimises contamination problems from any of the material being sputtered from the scan wheel during processing.
By minimising the risk of the ion beam contacting the side faces of the spoke arms, it can be unnecessary to provide any such compatible coating on the side faces, so that only the front face of the spoke arms need be coated.
Preferably, each spoke arm comprises a first radial flange piece extending generally in the plane of rotation of the scan wheel and providing said spoke arm front face, and at least one radial web piece extending generally in an axial plane and connected to a rear face of the flange piece. In a particular embodiment, each spoke arm includes two said radial web pieces having a space between them in the direction of rotation.
This construction provides excellent strength and rigidity with reduced mass.
The radial web pieces have respective outwardly directed side faces which form said major parts of said spoke arm side faces.
Preferably, the major parts of the spoke arm side faces do not extend outwards beyond a rearward projection of said front face which, in cross-section, tapers symmetrically inwards by 10xc2x0, and more preferably by 12xc2x0-13xc2x0, on each side relative to said axis of rotation. This increased tapering effect allows a maximum implant angle of up to plus or minus 10xc2x0 without the ion beam impinging on the side faces of the spoke arms.
In a preferred embodiment, each said wafer support element front face is adapted to mount a semiconductor wafer thereon in a wafer mounting plane defining a wafer axis normal to said wafer mounting plane, and, at least over said predetermined radial distance inwards from an innermost edge of the respective wafer support element, each spoke arm front face has no region facing towards the respective said wafer axis. This arrangement is preferably, though not necessarily, used with the tapered profile feature described above.
By ensuring that the spoke arm front faces do not face towards the wafer axis over the predetermined radial distance, the danger of contamination of the wafer during over scanning, i.e. when the ion beam impinges on the spoke arm front faces, is reduced. Material sputtered from the front faces of the spoke arms by the impinging ion beam have an angular distribution relative to the surface of the front face with a maximum at the normal to the front face. Therefore, if the front face of the spoke arm faces towards the axis of the wafer, sputtered material tend to be ejected into the region in front of the wafer, thereby increasing the probability of contaminating the wafer surface.
In fact, it is preferable if said spoke arm front faces face away from the respective said wafer axes.
In a further embodiment, at least over said predetermined radial distance, said spoke arm front faces are angled towards said wheel axis. Then, any discrete sheet of coating material, as contemplated in U.S. Pat. No. 4,937,206, applied to the front faces of the spoke arms is retained firmly against the front faces by centrifugal force when the scan wheel is spinning during an implant process. In this way, there is reduced risk of such a coating material becoming detached during processing and the problem of securing the coating material to the spoke arm front faces is alleviated. Also the centrifugal force improves thermal contact between the coating material and the arm.
This orientation of the spoke arm front faces towards the wheel axis is preferably, though not necessarily, used with one or both of the above mentioned tapered spoke arm cross-section feature and contamination reduction feature.
In practice the spoke arm front faces may be angled relative to a radial plane at between 3xc2x0 and 10xc2x0.
In a further preferred embodiment, at least over said predetermined radial distance, said spoke arm front faces are planar. This planar construction greatly facilitates the ability to affix replaceable screening coatings to the front faces of the spoke arms. Once again this planar spoke arm front face feature is preferably, though not necessarily, used with any one or a combination of the aforementioned features.
Conveniently, each said wafer support element front face is adapted to mount a semiconductor wafer thereon in a wafer mounting plane facing towards said wheel axis. This arrangement ensures that centrifugal force will assist in retaining the semiconductor wafer in position on the wafer support element during rotation of the scan wheel.
The aforementioned predetermined radial distance inwards along each spoke arm from an innermost edge of the respective wafer support element is preferably greater than about 50 mm. The important criterion is for this distance to be greater than the beam dimension in this direction so that the spoke arms have the described characteristics everywhere that the ion beam is likely to impinge along the radial length of the spoke arms. A typical ion beam width in the radial direction of the spoke arm is 50 mm. In practice, the predetermined radial distance along the spoke arms may be between 70 and 110 mm.
In a further preferred embodiment, each said wafer support element front face has a radially innermost edge and said spoke arm front face has a portion immediately radially inside said innermost edge of the respective wafer support element front face which is spaced at least 1 cm behind said innermost edge in the direction of said axis.
This feature also assists in reducing the likelihood of contamination of the semiconductor wafers due to materials sputtered from the spoke arms during processing. Preferably the spacing is as great as possible and in one example about 1.5 cms.
This spacing feature may also be used independently but preferably in addition to one or more of the previously described features.